1. Field of the Invention
The present invention relates generally to a beam index color cathode ray tube, and more particularly to a beam index color cathode ray tube which can reproduce color with high fidelity.
2. Description of the Prior Art
In a prior art beam index color cathode ray tube, a single electron beam scans a target screen which consists of triads of parallel red, green and blue vertical color phosphor stripes sequentially repeating across the screen. The color phosphor stripes are sequentially scanned by a scanning electron beam which crosses the color phosphor stripes horizontally in sequence from one side of the screen to the other. Index phosphor stripes are provided on the inner surface of the screen parallel to and in known relationship to the color phosphor stripes. As the electron beam scans horizontally across the screen, it excites the index phosphor stripes into producing a light signal behind the screen. Such light signal is detected by a photodetector to produce an index signal which has a known relationship to the instantaneous position of the electron beam on the screen.
The index signal is used to control the modulation of the electron beam such that the electron beam is density modulated with the red primary color signal when the beam scans across a red phosphor stripe, with the green primary color signal when the beam scans across a green phosphor stripe and with the blue primary color signal when the beam scans across a blue phosphor stripe, respectively.
FIGS. 1-3 show arrangements of index phosphor stripes on the inner surface of the screen and positioned between the adjacent color phosphor stripes in prior art color cathode ray tubes. In FIG. 1, the pitch P.sub.I of the index phosphor stripes 11 is the same as, or an integral multiple of, the pitch P.sub.T of each triad of red, green and blue color phosphor stripes R, G and B. The positional relationship between the index phosphor stripes 11 and the respective triads of red, green and blue color phosphor stripes R, G and B is fixed and readily determined from the index signal and hence color synchronization should be achievable in a relatively simple manner.
However, since the relative positions of the index phosphor stripes 11 and the red, green and blue phosphor stripes R, G and B is fixed, any phase shift in the index signal produced, for example, by color modulation of the scanning beam, results in faulty color synchronization and degrades color reproduction fidelity. This is especially noticeable in the reproduction of highly saturated color since the high electron beam current for a particular highly saturated color creates an apparent shift in phase of the index signal.
Phase shift due to color modulation of the scanning beam arises due to conditions illustrated in FIGS. 4A-4C. For purposes of description, it is assumed that the electron scanning beam is a point. In FIG. 4A the electron beam current is constant as the beam scans across index stripe 11. The resulting index signal rises steeply as the scanning beam touches stripe 11, remains approximately constant as the scanning beam scans across the stripe and descends sharply as the scanning beam leaves the stripe. As will be explained, the phase of the index signal is used to control the switching of color modulation signals between the red, green and blue colors such that a red color modulating signal controls the electron beam as it crosses a red color phosphor stripe, a green color modulating signal controls the electron beam as it crosses a green color phosphor stripe and the blue color modulation signal controls the electron beam as it crosses a blue color phosphor stripe. If there is an error in detecting the phase of the index signal, the phase of the color control signals can also be shifted such that, for example, when the electron beam scans the green color phosphor stripes, the electron beam density may be affected by the red or blue color signals.
The manner in which phase shifts of the index signal due to color modulation can be detected is shown in FIGS. 4B and 4C. In FIG. 4B, the beam current is assumed to be increasing as the beam scans across the index stripe 11, and the resulting index signal slopes upward to the right. When detecting the phase of the resulting index signal in FIG. 4B, the phase appears to be shifted forward in time, as indicated by the arrow. Similarly, when scanning with a beam which is decreasing in intensity as shown in FIG. 4C, the resulting index signal slopes downward to the right and appears to be retarded in time, as indicated by the arrow.
FIG. 5 shows an example of beam current variation in relation to the scanning of the color phosphor stripes and index stripes of a beam index cathode ray tube. In the illustrated example, the red signal is assumed to have a higher amplitude than the green, which, in turn has higher amplitude than the blue modulating signal. In addition, the modulating signals are shown to make a sharp transition from one to the other as the beam scans midway between color phosphor stripes. Such variation of the beam current as the beam scans the index phosphor stripes 11 produces the index signal shown on FIG. 5 to have a higher amplitude at the left-hand portion corresponding to the beam current for the red color and a lower portion at the right-hand portion corresponding to the beam current for the green color. As indicated by the arrows, this produces an apparent retarding phase shift. Synchronizing circuits responding to this index signal would incorrectly shift the transition point between color leftward and this would result in faulty color synchronization. The faulty color synchronization would, of course, change as the ratio of the red and green color modulating signals was varied.
Because of the phase shift due to color modulation of the scanning beam, a one-to-one relationship between index phosphor stripes 1a and color phosphor stripe triads has been considered undesirable. Therefore, the arrangements in FIGS. 2 and 3 are frequently used. The pitch P.sub.I of the index phospor stripes 11 is selected to be a non-integral multiple of the pitch P.sub.T of the triads of color phosphor stripes. Thus, pitches P.sub.I of 2/3, 4/3 or generally (3n.+-.1)/3(where n is 0, 1, 2, . . . ) of the pitch P.sub.T of the triads of red, green and blue color phosphor stripes R, G and B may be used. With the foregoing arrangement, the positional relationships between the index phosphor stripes 1a and the triads of red, green and blue color phosphor stripes R, G and B are varied sequentially across the image area so that a phase shift in the index signal due to color modulation does not appear uniformly across the screen and hence the color synchronization is considerably improved.
The variation in positional relationships across the screen between the index phosphor stripes 11 and the triads of red, green and blue color phosphor stripes R,G and B, requires a relatively complex synchronization technique to establish color synchronization.
Even with a non-integral relationship between the triads of color phosphor stripes and the index stripes, although this arrangement improves color synchronization, spurious phase shifts in the index signal due to color modulation of the scanning beam continue to cause color phase errors in color reproduction.
Although the foregoing description was based on the assumption that the electron beam is a point, the same results are obtained with a real electron beam which has finite dimensions.
A further reason for faulty color synchronization is the time delay inherent in practical circuits between detecting an index signal, determining that a phase error exists, generating a correction signal and applying the correction signal to the beam index cathode ray tube. Due to the time delay involved, when the frequency of the index signal drifts, its phase also shifts. Consequently, when the scan speed of the electron beam changes from point to point across the image area, the resulting change in frequency of the index signal produces a phase shift which affects color synchronization which varies across the screen according to speed of the scanning beam.